$6.0$ litres of a gaseous mixture containing propane and butane on complete combustion at $25°\mathrm{C}$ produced $20$ litres of ${\mathrm{CO}}_2.$. Then the composition of the given mixture is __________

$50 \mathrm{mL}$ of $0.2 \mathrm{N} \mathrm{HCl}$ is titrated against $0.1 \mathrm{N} \mathrm{NaOH}$ solution. The titration is discontinued after adding $50 \mathrm{mL}$ of $\mathrm{NaOH}$ solution. The remaining titration is completed by adding $0.5 \mathrm{N} \mathrm{KOH}$ solution. What is the volume of $\mathrm{KOH}$ solution required for completing the titration ?